Apple could sell 1 million watches opening weekend, and barely move the needle on Apple's revenue

Apple will sell over one million Apple Watches over the opening weekend, according to Piper Jaffray analyst Gene Munster.

By way of comparison, the iPhone 6 and 6 Plus sold over 10 million units in their first three days on the market last September, Apple says.

Here's his reasoning:

1 million total Watch units in opening weekend would imply an attach rate of less than 1% of total iPhone users (close to half a percent). We believe given interest in the product and our prior survey work, a sub 1% attach rate in the opening weekend is realistic.

Munster expects 300,000 watches to sell in the first 24 hours — an 8% attach rate of iPhone 6/6 Plus users, who are Apple's presumably most loyal customers — and 8 million by the end of the year, accounting for $4.4 billion in revenue, or about 2% of Apple's total.

That 8 million figure is more bearish than most other estimates, which range between 10 and 30 million, with average consensus landing around 14 million. Munster does believe Apple will sell 4o to 50 million by the end of 2017, meaning about 10% of the iPhone user base will have bought one.